Examples of conventional heat exchanger are disclosed in Japanese Patent No. 2835286, Japanese laid-open patent publication No. 62-288446 and Japanese examined utility model publication No. 6-8442. In the heat exchanger disclosed in these documents, a helical tube body of a water tube is accommodated in a casing into which combustion gas is to be introduced. Heat is recovered from the combustion gas by the helical tube body, so that water supplied into the water tube is heated. As compared with a straight tube body, such a helical tube body has a larger heat transfer area. Therefore, with the heat exchanger, the amount of heat recovery can be increased while the number of water tubes is reduced.
However, the above-described conventional structure has the following problems.
As schematically shown in FIG. 9, the helical tube body 9 of the water tube of the conventional structure includes a plurality of loops 90 continuously connected to each other and arranged in the axial direction (the direction in which the central axis of the helical tube body 9 extends). Each of the loops 30 is inclined at a predetermined angle • throughout the entire length thereof. With this structure, the height h1 of a half region of one loop 90 is given by h1=L• tan •, and the pitch p1 of the helical tube body 9 is given by p1=2•h1=2•L• tan •, where L is the diameter or width of the helical tube body 9. Thus, the pitch p1 is relatively large. To increase the amount of heat to be recovered by the water tube, the number of loops 90 needs to be increased. However, since the dimensions h1 and p1 are relatively large, the overall height h of the helical tube body 9 becomes considerably large when the number of loops 90 is increased. Thus, the conventional structure has a drawback that the overall height of the helical tube body 9 becomes considerably large when the number of loops 90 is increased to increase the amount of heat recovery.
In cold season, for example, the use of a water heater provided with a heat exchanger may be stopped for a long time. In such a case, to prevent the inside of the water tube from freezing, draining of water from the water tube may be performed. Therefore, it is desired that the above-described problem as to the height increase is solved without making the draining of the water tube difficult.